7-digit frequency meter/digital scale


The construction is based on Microchip Application notes No 590 (LED display multiplexing) and 592 (frequency meter).
The mathematics is mainly original product of my mind.
The device was originally developed for compact LED based multiplexed displays used in calculators. Standard (nonmultiplexed) LED displays can be used too, they must have hard vired corresponding pins.

The prototype was developed with common cathode LED display.
It is 7-digit counter (frequency meter) counting up to 35 MHz. The decimal point is after MHz digit, but can be set at any position. Also two decimal points can be set. Or after each number... ;-)

Power consumptions with small calculator display: 2.5V/9mA, 3V/13mA, 5V/35mA.

Hardware is very simple:

It contains :

  • PIC 16F84 or PIC 16C84 (I have used ...-04 variant up to 10 MHz Xtal with good success),
  • 4051 (BCD -> 1 of 8 decoder)
  • 8 NPN low power Si transistors,
  • 7-digit calculator display (common cathode),
  • some resistors, capacitors and 2 switching diodes

You will need also include files.

Note:
"Calculator display" means minimum 7 digit LED multiplexed display. Both common cathode and common anode can be used. Software is written for common cathode display. For common anode displays it requires very slight software and hardware modification.

The counter uses internal prescaler of PIC as low byte of counter, TMR0 as middle byte and some register as high byte of counter.

Some ideas were taken from "Simple low-cost digital frequency meter using a PIC 16C54" (frqmeter.asm) written by James Hutchby, MadLab Ltd. 1996